Combustors are commonly used in industrial and power generation operations to ignite fuel to produce combustion gases having a high temperature and pressure. For example, turbo-machines such as gas turbines typically include one or more combustors to generate power or thrust. A typical gas turbine includes an inlet section, a compressor section, a combustion section, a turbine section, and an exhaust section. The inlet section cleans and conditions a working fluid (e.g., air) and supplies the working fluid to the compressor section. The compressor section increases the pressure of the working fluid and supplies a compressed working fluid to the combustion section. The combustion section mixes fuel with the compressed working fluid and ignites the mixture to generate combustion gases having a high temperature and pressure. The combustion gases flow to the turbine section where they expand to produce work. For example, expansion of the combustion gases in the turbine section may rotate a shaft connected to a generator to produce electricity.
The combustion section may include one or more combustors annularly arranged between the compressor section and the turbine section, and the temperature of the combustion gases directly influences the thermodynamic efficiency, design margins, and resulting emissions of the combustor. For example, higher combustion gas temperatures generally improve the thermodynamic efficiency of the combustor. However, higher combustion gas temperatures also promote flame holding conditions in which the combustion flame migrates towards the fuel being supplied by nozzles, possibly causing accelerated damage to the nozzles in a relatively short amount of time. In addition, higher combustion gas temperatures generally increase the disassociation rate of diatomic nitrogen, increasing the production of nitrogen oxides (NOX) for the same residence time in the combustor. Conversely, a lower combustion gas temperature associated with reduced fuel flow and/or part load operation (turndown) generally reduces the chemical reaction rates of the combustion gases, increasing the production of carbon monoxide and unburned hydrocarbons for the same residence time in the combustor.
In a particular combustor design, the combustor may include a cap assembly that extends radially across at least a portion of the combustor, and one or more fuel nozzles may be radially arranged across the cap assembly to supply fuel to the combustor. The combustor may also include at least one annular liner that extends downstream from the cap assembly. The liner at least partially defines a combustion chamber within the combustor. The liner further defines a hot gas path that extends between the combustion chamber and an inlet to the turbine. The fuel nozzles may include swirler vanes and/or other flow guides to enhance mixing between the fuel and the compressed working fluid to produce a lean fuel-air mixture for combustion. The swirling fuel-air mixture flows into the combustion chamber where it ignites to generate the hot combustion gases. The hot combustion gases are routed through the hot gas path to the inlet of the turbine.
The combustor may further include one or more fuel injectors circumferentially arranged around the combustion chamber and/or the liner to supply additional fuel for combustion to the combustion chamber and/or to the hot gas path generally downstream from the combustion chamber. This system and method for operating a combustor is commonly referred to in the power generation industry as Late Lean Injection or LLI. The additional fuel supplied by the fuel injectors increases the firing temperature of the combustor without producing a corresponding increase in the residence time of the combustion gases inside the combustion chamber.
Although generally effective at enabling higher operating temperatures, the overall effectiveness of the LLI is at least partially dependent upon how well the fuel-air combination that flows from the injector mixes with the swirling fuel-air mixture in the combustion chamber and/or with the hot combustion gases flowing through the liner generally downstream from the combustion chamber. For example, enhanced mixing of the fuel-air combination from the injector with the swirling fuel-air mixture in the combustion chamber and/or with the hot combustion gases flowing through the liner reduces peak flame temperature within the combustor, thereby reducing NOx levels. As a result, a system for supplying fuel to a combustor that enhances mixing of the fuel-air combination that flows from the fuel injectors circumferentially arranged around the combustion chamber and/or the liner with the swirling fuel-air mixture in the combustion chamber and/or with the hot combustion gases flowing through the liner would be useful.